The invention relates to fine wire of a gold alloy containing mischmetal and platinum or palladium (or a mixture thereof) for bonding to semiconductor devices to form an electrical connection to the semiconductor, a method for manufacture thereof and the use thereof.
Wires suitable for bonding to semiconductor devices, also known as bonding wires, must have good electrical characteristics and good mechanical strength values. The diameter of the wires is selected to match the requirements of the application and can range from about 10 to 200 micrometers and is usually about 20 to 60 micrometers.
Bonding wires are often made of high-purity gold or of gold alloys containing rare earth metals.
For example, DE 16 08 161 C teaches that an alloy of gold and 0.001 to 0.1% of one or more rare earth metals, especially in the form of mischmetal or yttrium, can be used to make lead wires in integrated circuits. This gold alloy containing small quantities of rare earth metals or yttrium has much better strength and elongation behavior at heating temperatures up to 500.degree. C. without experiencing substantial influence on other characteristics of the gold such as hardness, chemical stability or electrical resistance.
Gold-rare earth metal alloys for bonding wires are also described in DE 32 37 385 A (U.S. Pat. No. 4,885,135), DE 39 36 281 A (U.S. Pat. No. 4,938,923), JP 5-179375 A, JP 5-179376 A, JP 6-112258 A, EP 0 743 679 A and EP 0 761 831 A.
DE 32 37 385 A relates to a gold-alloy fine wire with high tensile strength comprising a gold alloy containing 0.0003 to 0.01 wt % of rare earth metal, especially cerium, and also germanium, beryllium and/or calcium as appropriate.
DE 39 36 281 A describes a gold wire, for connecting a semiconductor device, of high-purity gold alloyed with small quantities of lanthanum, beryllium, calcium and platinum group elements, especially platinum and/or palladium.
JP 5-179375 A and JP 5-179376 A relate to gold-alloy fine wires for bonding which comprise high-purity gold and 0.0003 to 0.005 wt % of aluminum or gallium, 0.0003 to 0.003 wt % of calcium and 0.0003 to 0.003 wt % of yttrium, lanthanum, cerium, neodymium, dysprosium and/or beryllium.
The bonding wire known from JP 6-112258 A, cited in Chemical Abstracts Vol. 121, 89287m, comprises a gold alloy containing 1 to 30% of platinum and 0.0001 to 0.05% of scandium, yttrium and/or rare earth metal and also 0.0001 to 0.05% of beryllium, calcium, germanium, nickel, iron, cobalt and/or silver as appropriate.
A bonding wire of a platinum-containing gold-rare earth metal alloy is also proposed in EP 0 743 679 A. The alloy comprises gold and small quantities of platinum (0.0001 to 0.005 wt %), silver, magnesium and europium, and can also contain, for example, cerium in a quantity of 0.0001 to 0.02 wt %.
A fine wire of a gold-rare earth metal alloy containing platinum and/or palladium is described in EP 0 761 831 A. The alloy comprises 0.1 to 2.2 wt % of platinum and/or palladium, 0.0001 to 0.005 wt % of beryllium, germanium, calcium, lanthanum, yttrium and/or europium, the remainder being gold. The wire is made by melting the alloy-forming elements in a crucible, progressively cooling the molten alloy contained in the crucible from bottom to top to obtain a casting (ingot), and thereafter rolling, drawing and annealing. It has an elongation of 3 to 8% and a Young's modulus of 6800 to 9000 kgf/mm.sup.2.
A bonding wire of gold, 0.1 to 0.8 wt % of platinum and 0.0003 to 0.01 wt % of calcium, beryllium, germanium, rare earth metal, strontium, barium, indium, tin and/or titanium is known from JP 7-335685 A (Patent Abstracts of Japan).
JP 8-293515 A (Patent Abstracts of Japan) also relates to bonding wires of gold-platinum alloys. The gold alloys contain in addition to gold 0.04 to 1.5 wt % of platinum, 0.004 to 0.06 wt % of yttrium, calcium, lanthanum and/or cerium and also 0.0005 to 0.05 wt % of aluminum and/or indium as appropriate.
JP 6-112254 A (Patent Abstracts of Japan) specifies the manufacture of a bonding wire of a mixture of gold, 0.1 to 1 wt % of platinum and 0.0001 to 0.005 wt % of iron, silicon, beryllium, calcium, germanium, yttrium, scandium and/or rare earth metals. The mixture is melted and cast. This is followed by rolling to shape, annealing and wire drawing to obtain a wire with a diameter of 25 micrometers. The strength of the bonding wire is increased by the addition of platinum, whereby failures in a high temperature test become less frequent.
JP 6-112256 A (Patent Abstracts of Japan) specifies the manufacture of a bonding wire of a mixture of gold, 0.003 to 0.1 wt % of palladium, platinum, rhodium, iridium, osmium and/or ruthenium and 0.0001 to 0.05 wt % of scandium, yttrium and/or rare earth metals. The mixture is melted and cast. This is followed by rolling to shape, annealing and wire drawing to obtain a wire with a diameter of 25 micrometers. The strength of the bonding wire is increased by the addition of the platinum group metals, whereby failures in a high temperature test become less frequent.
JP 2-91944 A (Patent Abstracts of Japan) relates to a gold fine wire suitable for gold bumps comprising a gold alloy, which in addition to gold contains 0.5 to 10 wt % of palladium and 0.0001 to 0.02 wt % of lanthanum, cerium and/or calcium.
DE 44 42 960 C relates to bumps for mounting flip chips and methods for making the same. The bump cores can be made mechanically as ball bumps using a wire material of 98% gold and 2% palladium.
The requirements for selecting bonding wires include not only special chemical and physical properties but also, in particular, the highest possible strength for given elongation.
Proceeding from DE 16 08 161 C, the object of the invention is therefore to provide fine wire of the type characterized in the introduction, comprising a gold alloy containing mischmetal and having the best possible strength-to-elongation ratio. Another object is to provide a cost-effective, continuous method for making the fine wire. The fine wire is intended to be suitable both for wire bonding and also for making ball bumps for flip-chip technology, as is described, for example, in DE 44 42 960 C.